1. Field of the Invention
The present invention is directed to a method for characterizing polarization transformers and, in particular, polarization transformers traversed by optical signals.
2. Description of the Related Art
International Patent Application PCT/DE 99/03875 discloses polarization transformers and compensators of polarization mode dispersion wherein redundant polarization setting elements that are not required for the actually desired function are utilized for generating control signals that are free of d.c. parts on chronological average. This serves the purpose of avoiding what is referred to as d.c. drift that makes the optical properties of polarization setting elements time-variant and, thus, unpredictable. Despite the described procedure it does not seem certain that d.c. drift can be thereby completely avoided. Moreover, polarization transformers are also subject to other modifications, for example temperature-dependent modifications, of their optical and electro-optical properties. If the properties of a polarization transformer that are present at the moment could be measured during operation, then unwanted variations such as d.c. drift could be neutralized by adapting parameters such as, for example, offset voltages.
In the reference IEEE J. Lightwave Techn. 6(1988)7, pp. 1199-1208, at pages 1205-1206 is disclosed a characterization method for electro-optical wave plates. This and other known characterization methods cannot be utilized during the operation of polarization transformers or PMD (Polarization Mode Dispersion) compensators. The reason for this is that specific operating parameters must be set for the characterization, so that the required degrees of freedom would no longer be available for the actual polarization transformation or PMD compensation. Bit errors due to faulty PMD compensation, for example, would thereby arise in the optical data transmission.
It is therefore an object of the invention to improve the d.c. drift suppression or other aging phenomena suppression in the form of a method for characterizing polarization transformers.
This object is achieved by a method for characterizing a polarization transformer that is traversed by an optical signal, including the steps of providing a first polarization transformer to be characterized and a further polarization transformer that lies before or after the first transformer in a light path of the optical signal and connected so that both are traversed by the optical signal, and, during the characterization of the first transformer, the further polarization transformer at least approximately assures the implementation of the desired polarization transformation or compensation of polarization mode dispersion of the optical signal.
Advantageous developments are provided by a method further providing that a plurality of polarization transformers are characterized by alternating characterization of one polarization transformer and implementation of a desired polarization transformation by this polarization transformer. Preferably, one polarization transformer acts as an elliptical retarder with a delay. At least a part of this polarization transformer acts as a mode converter with a selectable delay and selectable orientation, i.e. phase difference, between the converted and non-converted signals. In one embodiment, at least a part of this polarization transformer acts as a Soleil-Babinet analog with selectable delay and selectable orientation with mode conversion between the TE and TM modes. According to a development of the invention, at least a part of this polarization transformer acts as a Soleil-Babinet compensator with a selectable delay and selectable orientation with mode conversion between circular polarizations. A mode converter to be characterized may be operated with full mode conversion, i.e. a delay equal to an odd-numbered multiple of the value xcfx80 and variable orientation. A mode converter functionally following this mode converter to be characterized at least approximately experiences changes of its orientation that correspond to twice the change of the orientation of the mode converter to be characterized. A polarization transformer functionally preceding this mode converter to be characterized at least approximately experiences no change of its control parameters.
The polarization transformer to be characterized in an embodiment is operated with a delay equal to an even-numbered multiple of the value 2xcfx80 and variable orientation. The polarization transformer to be characterized may be operated with variable delay. As a preferred development, changes of the polarization transformation of the optical signal caused by parameter variation of a polarization transformer to be characterized are detected by a detection means. Specifically, a change of other parameters of a polarization transformer to be characterized is undertaken as a function of an output signal of this detection means. This ensues for the purpose of minimizing an output signal of this detection means, or for the purpose of maximizing an output signal of this detection means. The output signal of this detection means can be supplied to a controller and another polarization transformer to at least approximately compensate these changes of the polarization transformation. A change of other parameters of a polarization transformer to be characterized is undertaken as a function of the change of parameters of a further polarization transformer. In a particular embodiment, this ensues for the purpose of minimizing the change of parameters of a further polarization transformer or, alternately, this ensues for the purpose of maximizing the change of parameters of a further polarization transformer.
A polarization transformer to be characterized is part of a polarization transformer that can generate a fixed polarization from every arbitrary polarization of the optical signal or serves for the receiver-side PMD compensation; and potentially existing polarization transformers functionally preceding the polarization transformer to be characterized lie in the beam path preceding the polarization transformer to be characterized, whereas potentially existing polarization transformers that functionally follow lie in the beam path behind the polarization transformer to be characterized.
A polarization transformer to be characterized is part of a polarization transformer that can generate any arbitrary polarization from a fixed polarization of the optical signal or serves for transmission-side PMD compensation; and potentially existing polarization transformers functionally preceding the polarization transformer to be characterized lie in the beam path following the polarization transformer to be characterized, whereas potentially existing polarization transformers that functionally follow lie in the beam path preceding the polarization transformer to be characterized.
In the method according to the invention, the first polarization transformer can be operated with voltages opposite those for the implementation of desired polarization transformation at times wherein the further polarization transformer at least approximately assures the implementation of the desired polarization transformation or compensation of polarization mode dispersion of the optical signal.
The solution of the problem is comprised in the characterization of polarization transformers or PMD compensators during their operation that are to be equipped with redundancy for this purpose. Some of the degrees of freedom thereby serve the desired control function, whereas some other, redundant degrees of freedom serve for the characterization. The architecture of the polarization transformer and the control voltages are selected such that the control function of the polarization transformer is not negatively affected.
In the inventive method, for example, a polarization transformer or PMD compensator is employed for normal control purposes and modifications of the desired polarization transformation during the characterization of a part of the polarization transformer are potentially intercepted by modifications of control parameters of other parts of the polarization transformer with the assistance of an adequately fast polarization control means. The size and direction or, too, the absence of these changes of control parameters is then a criterion for polarization transformations of the polarization transformer to be characterized. Alternatively thereto, the changes of the desired polarization transformation during the characterization of a part of the polarization transformer can be merely detected, particularly when the aim of the characterization is to minimize these changes. There are a great number of exemplary embodiments for this, but all of these work according to the same principle.
For example, a polarization transformer for the transformation of an arbitrary polarization in a fixed, linear polarization state in lithium niobate (LiNbO3) with an X-section and a Z propagation direction can be composed of 8 cascaded electro-optical wave plates that can respectively assume a maximum delay of at least xcfx80/2. The first 4 wave plates work as a normal polarization transformer P1. The following 4 wave plates form a polarization transformer P2 that is characterized. To this end, those control voltages of the polarization transformer P2 are sought that allow an endless phase shift of a specific output polarization compared to the one orthogonal thereto. The search criterion is thereby that the control voltages applied to the wave plates of P1 for the desired polarization transformationxe2x80x94and, potentially, of further polarization transformers when employed as part of a PMD compensatorxe2x80x94must be subject to only specific or optimally slight or even no changes. Subsequently, the transformers P1 and P2 interchange function, so that the transformer P1 is characterized and the transformer P2 undertakes the normal polarization transformation.